Multi-color emitting phosphors will be potential in the fabrication of solid state lighting devices. Color-tunable Sr6YSc(BO3)6:Ce3+,Tb3+ phosphors were synthesized by a high temperature solid-state reaction, and the crystal structure and luminescence properties were investigated in detail. The photoluminescence excitation spectrum of Sr6YSc(BO3)6:Ce3+ show that the excitation peaks from 200 to 400 nm are attributed to the characteristic 4f-5d transitions of Ce3+, and the broad-band blue emission can be also found from the as-synthesized Sr6YSc(BO3)6:Ce3+. Under the excitation of near ultraviolet (n-UV) light, Sr6YSc(BO3)6:Ce3+,Tb3+ phosphors exhibit not only a broad blue emission band originating from the f–d transition of Ce3+ ion but also a group of sharp characteristic green emission lines from the f–f transition of Tb3+ ion, respectively. The excitation spectra monitored at 544 nm emission of Tb3+ consists of the characteristic excitation bands originating from Ce3+ and Tb3+ ions, which proves the occurrence of the energy transfer between Ce3+ and Tb3+. The energy transfer behaviors in Sr6YSc(BO3)6:Ce3+,Tb3+ phosphors is also investigated by the lifetime measurement. The above results indicate that Sr6YSc(BO3)6:Ce,Tb can act as a potential candidate for near-UV-pumped light emitting diodes.
In recent years, along with the energy shortage and global warming, the development of energy saving products in the lighting field, such as white light-emitting diode (WLED), has attracted great attention . Due to the potential applications in indicators, green architectural lighting, automobile headlights, backlights, and general illuminations, WLEDs have become a kind of daily lighting source with excellent properties such as high efficiency, good light stability, long operational lifetime, and environmentally friendly characteristics [2–5]. Furthermore, many efforts have been made in the search of down-converting phosphors converting ultraviolet (UV) or blue light into a combination of red-green-blue light in order to obtain white light emission . Commercial WLEDs lamp is commonly fabricated by using a blue InGaN LED chip and the yellow-emitting Y3Al5O12:Ce3+ (YAG:Ce) phosphor. However, such WLEDs have a poor color rendering index (CRI) and a high correlated color temperature (CCT) [7, 8] because of lacking a red component. However, WLEDs can be also fabricated by pumping blue, green and red phosphors coated on a near-UV (n-UV) LED chip which has a high CRI . For this reason, it is necessary to develop novel multi-color emission phosphors in the field of optical materials.
Recently, borates compounds attracted much attention due to their excellent optical properties, low synthesis temperature, less expensive raw materials, and high physical chemistry stability [10, 11]. Among them, the new compounds with the general formula of A6MM(BO3)6, (A = Sr; M = Gd, Y; M = Al, Ga, In, Sc and Y) has been reported previously [12, 13]. In this work, Sr6YSc(BO3)6 was selected as the host material, Ce3+, Tb3+ singly doped and co-doped Sr6YSc(BO3)6 phosphors were firstly reported. It is well known that Ce3+ ions acts as a good sensitizer in co-doped materials, and transfers a part of energy to the activator ions , such as Eu2+, Tb3+ and Mn2+[14–16]. As for the Ce3+-Tb3+ couples with fixed Ce3+ content, the Tb3+ emission from 5D3 level will be quenched and 5D4 emission will increase gradually with increasing Tb3+ concentration . Herein, the novel color-tunable Sr6YSc(BO3)6:Ce3+,Tb3+ phosphors have been obtained via the traditional solid-state method. Blue to blue-green emission can be realized in this series of phosphors by changing the Tb3+ concentration. Luminescent properties and energy transfer process between the sensitizer (Ce3+) to activator (Tb3+) in the Sr6YSc(BO3)6 host have been studied, and the energy transfer mechanism is also investigated.
A series of Sr6YSc(BO3)6:Ce3+,Tb3+ phosphors were prepared by using the conventional high temperature solid state reactions. SrCO3 (99.5%), Y2O3 (99.99%), Sc2O3 (99.99%), H3BO3 (99.5%), CeO2 (99.99%) and Tb4O7 (99.99%) were used as the starting materials. Stoichiometric amounts of the starting materials were thoroughly mixed and ground thoroughly in an agate mortar. Then, the mixture was transferred into an alumina crucible and calcined in a muffle furnace at 1100°C for 6 h under CO reducing atmosphere. The as-prepared samples were reground into powder for measurement at room temperature.
The crystal structures of the samples were checked by X-ray powder diffraction (SHIMADZU, XRD-6000, 40 kV and 30 mA, Cu Kα λ = 0.15406 nm). Photoluminescence excitation (PLE) and emission (PL) spectra were performed by using a JOBIN YVON FluoroMax-3 fluorescence spectrophotometer with a photomultiplier tube operating at 400 V, and a 150 W Xe lamp used as the excitation lamp. The decay time were carried on using a spectro-fluorometer (HORIBA, JOBIN YVON FL3-21), and the 250 nm pulse laser radiation (nano-LED) was used as the excitation source. And the CIE coordinates were calculated based on the photoluminescence spectra between 375 and 650 nm.
Results and discussions
Figure 1 illuminates the XRD patterns of as-prepared Sr6YSc(BO3)6:0.005Ce3+, Sr6YSc(BO3)6:0.005Ce3+,0.03 Tb3+ and Sr6YSc(BO3)6:0.005Ce3+,0.20 Tb3+ phosphors. All the peaks match well with the standard data of JPCDS card no. 79–2382 (Sr6YSc(BO3)6), and no other crystalline phase can be detected, indicating that all the as-synthesized samples are of single phase. These results illuminate that the doped Ce3+ and Tb3+ ions were completely dissolved in the Sr6YSc(BO3)6 host lattice without any significant change in the crystal structure. Figure 2 shows the crystal structure of Sr6YSc(BO3)6, and yellow, blue, green polyhedrons represent YO8 dodecahedron, ScO8 dodecahedron, BO3 trihedron, respectively, which belongs to the trigonal space group R - 3(148), and the lattice parameters are a =12.284(1) Å, c = 9.268(2) Å and V = 1211.15(33) Å3. Sr2+, Y3+, Sc3+ and B3+ metal ions are surrounded by quantitative oxygens. Y3+ and Sc3+ ions are randomly located in eightfold dodecahedral [Y/ScO8] sites. The B3+ ions are in three fold trihedron [BO3] sites. The Sr2+ is connected to nine oxygen while four O (1) is only connected with B3+ ions with Sc is surrounded by three O (2) and the other oxygen namely O (3) relate to the Y3+ ions . It is well known that an acceptable percentage difference in ion radii between the doped and substituted ions must not exceed 30%. Herein, it is known that the ionic radius of 8-coordinated Y3+ is 1.019 Å, which is close to that of Ce3+ (1.143 Å) or Tb3+ (1.04 Å) while the ionic radius of 8-coordinated Sc3+ is 0.87 Å, which is too small for Ce3+ or Tb3+ to be substituted. Consequently, it is reasonable that the Ce3+ and the Tb3+ ions are completely incorporated into the host lattice by substituting for Y3+ sites in the Sr6YSc(BO3)6 host.
Figure 3 demonstrates the photoluminescence excitation (PLE) and emission (PL) spectra of Ce3+ and Tb3+ single doped and Ce3+,Tb3+ codoped Sr6YSc(BO3)6 phosphors. As shown in Figure 3(a), the excitation spectrum monitored at 544 nm for Sr6YSc(BO3)6:0.30 Tb sample exhibits a shoulder and a broad band centered at 241 nm and 280 nm, which correspond to the spin-allowed and spin-forbidden 4f8-4f75d transition of Tb3+ ions, respectively . The PL spectrum consists of emission lines at 411, 489, 544, 583 and 623 nm, which should be ascribed to the transitions from the 5D4/5D3 excited state to the 7FJ (J = 6–3) ground states of Tb3+. Due to a magnetic dipole allowed transition, the intensity of 5D4 → 7 F5 transition peaks at 544 nm is much higher than that of the other emission lines. The emission spectrum under the excitation of 365 nm of Sr6YSc(BO3)6:0.005Ce3+ (Figure 3(b)) has an asymmetric blue band ranging from 375 to 600 nm with a maximum at 423 nm which is due to the characteristic 5d1 → 4f1 transition of Ce3+. The PLE spectrum consists of three major peaks corresponding to the typical transition of Ce3+. Moreover, it can be seen from Figure 3(c) that the emission spectrum monitored at 365 nm of Sr6YSc(BO3)6:0.005Ce3+,0.20 Tb3+ phosphor exhibits not only the obvious blue band from Ce3+ but also a group of sharp characteristic green line from Tb3+. Compared with that of the Sr6YSc(BO3)6:0.005Ce3+ sample, one can find that their PLE spectra are similar monitored by the emission of 423 nm. By Monitoring the 544 nm emission of Tb3+, the excitation spectrum consists of both the excitation bands of Ce3+ ions and that of the Tb3+ ions. The observed results show significant contribution for the excitation of emission of Ce3+,Tb3+ codoped samples originating from f-d transition of Ce3+, which also provides an obvious evidence of the energy transfer from Ce3+ to Tb3+. Furthermore, PLE spectrum monitored at 544 nm emission for the Sr6YSc(BO3)6:0.30 Tb3+ sample shows some line-type excitation lines in the range of 300 and 500 nm, which are ascribed to the 4f → 4f electrons transition of Tb3+ ions (Figure 4). Compared with Figure 3(b), one can find the spectral overlap between the emission spectrum of Sr6YSc(BO3)6:0.005Ce3+ and the excitation spectrum of Sr6YSc(BO3)6:0.30 Tb3+, which means the possible energy transfer process. Figure 4 also gives the emission spectrum of Sr6YSc(BO3)6:0.005Ce3+,0.03 Tb3+ phosphor upon the excitation of 254 nm, which includes the broad band from Ce3+ and the narrow lines from Tb3+. Moreover, the spectral overlap of the excitation spectrum of Tb3+-doped samples and emission spectrum of Ce3+,Tb3+ codoped samples also appears suggesting the possible energy transfer process.
PLE and PL spectra of Sr6YSc(BO3)6:x Ce3+ phosphors are shown in Figure 5. Under the excitation of 365 nm, all of the emission spectra exhibit the similar profile with different relative intensities. The emission band gradually becomes broader and red-shifts with increasing Ce3+ concentrations. After Gaussian deconvolution, the broad emission band of Sr6YSc(BO3)6:0.01Ce3+ can be well decomposed into two components centered at about 416 and 455 nm. The excitation spectrum monitored at 419 nm shows a typical broad band in the range of 200–400 nm, which is ascribed to the 4f-5d transition of Ce3+.
The energy transfer process from Ce3+ to Tb3+ is also investigated by the decay curves of Ce3+ emission in codoped samples of Sr6YSc(BO3)6:0.005Ce3+,x Tb3+ (x = 0.00, 0.01, 0.03, 0.05, 0.10, 0.20, 0.30). Figure 6 exhibits the room temperature decay curves of Ce3+ emission at 419 nm under the excitation at 250 nm, and the lifetime values of Ce3+ in also given in Figure 6. These decay curves are analyzed and they can be fitted successfully based on the following second-order exponential equation:.
Here I and I0 correspond to the luminescence intensity at time at time t and initially, respectively. A1 and A2 are two constants which are related with the initial intensity, τ1 and τ2 are the lifetimes for the exponential components. By using these parameters, the average lifetime constant (τ*) can be calculated by the formula as follows,
The measured fluorescence lifetimes (τ*) were calculated to be 29.49, 29.37, 29.13, 28.66, 25.11, 21.89 and 1.37 ns for Sr6YSc(BO3)6:0.005Ce3+,x Tb3+ with x = 0.00, 0.01, 0.03, 0.05, 0.10, 0.20 and 0.30, respectively. With increasing Tb3+ concentration, the value of τ* is observed to decrease gradually. Therefore, the energy transfer efficiency from Ce3+ to Tb3+ was calculated by the following formula:
where τ0 and τS stand for the lifetimes of Ce3+ in the absence and the presence of Tb3+, respectively. As shown in the inset of Figure 6, the energy transfer efficiency (ηT) increases with increasing Tb3+ concentration. The energy-transfer efficiencies from Ce3+ to Tb3+ in Sr6YSc(BO3)6:0.005Ce3+,x Tb3+ phosphors are calculated to be 0, 4%, 1%, 3%, 15%, 26% and 95% corresponding to x = 0.00, 0.01, 0.03, 0.05, 0.10, 0.20 and 0.30. All the obtained results prove that the energy-transfer process from Ce3+ to Tb3+ in Sr6YSc(BO3)6 is very efficient. In order to obtain the color-tunable emission and further optimize the green emission of Tb3+ ions, Tb3+ concentration dependent PL spectra of Sr6YSc(BO3)6:0.005Ce3+,x Tb3+ phosphors are studied. The excitation and emission spectra of Sr6YSc(BO3)6:0.005Ce3+,x Tb3+ phosphors with different Tb3+ doping contents (x = 0, 0.01, 0.02, 0.05, 0.10, 0.20, 0.30) upon an excitation wavelength of 365 nm are given in the Figure 7. Sr6YSc(BO3)6:0.005Ce3+,x Tb3+ phosphors show blue and green emission bands centering at about 419 nm and 544 nm due to the variation of the relative doping concentrations. As we all know that the emission spectrum of Tb3+ ions shows a characteristic group of sharp lines corresponding to the transitions of 5DJ (J = 3–4) and 7FJ (J = 0–6) states. As seen in Figure 7(a), an obvious blue emission band and some green emission peaks are observed in the emission spectra of all the Sr6YSc(BO3)6:0.005Ce,x Tb samples under the 365 nm excitation. As also seen in Figure 7(b), for x ≤ 0.05, the intensities for Ce3+ blue emission at 421 nm increased abnormally. Simultaneously, the emission intensity about 419 nm and the characteristic emission of Tb3+ increases. With the different radiation excitations, the same emission spectral profile also can be found in Figure 4. At the same time, with the increase in the Tb3+ concentration (0.05 ≤ x ≤ 0.20), the intensity of blue emission at 419 nm decreased while the emission intensity of Tb3+ due to transitions 5D4 to 7FJ (J = 6–3) increased. As for the high Tb3+ doping concentration (x ≥ 0.20), all the emission intensities decrease because of the concentration quenching effect suggesting that the possible energy transfer process could be existed between Ce3+ and Tb3+. On the basis of the above spectral analysis, the excitation and emission mechanism of Ce3+ and Tb3+, and the energy transfer processes (Ce3+ → Tb3+) in this host is schematically shown in Figure 8. The energy transfer process taking place in the system can be explained as follows. When Ce3+ and Tb3+ are co-doped in the same host, it is accepted that Ce3+ ions can absorb the near ultraviolet light of 365 nm. And the energy transfer take place from 5d energy level of Ce3+ to the high excitation levels 5D3 of Tb3+, which relaxes to 5D4 later, which transitions to the low level of 7FJ (J = 6–3).
Figure 9 exhibits the CIE chromaticity diagram and the corresponding digital photographs of the Sr6YSc(BO3)6:0.005Ce3+ and Sr6YSc(BO3)6:0.005Ce3+,0.20 Tb3+ phosphor. The violet and blue light emission with CIE coordinates of (0.1682,0.0934) and (0.2156,0.3004) can be obtained, as shown in Figure 9. Table 1 gives a summary of the x and y values of CIE chromaticity coordinates for Sr6YSc(BO3)6:0.005Ce3+,x Tb3+ phosphors with different Tb3+ concentrations under the excitation of 365 nm. With increasing Tb3+ content, the chromaticity coordinates for Sr6YSc(BO3)6:0.005Ce3+,x Tb3+ could be tuned from blue (0.1682,0.0934) to blue-green (0.2243,0.3261) position by adjusting the doping content of Tb3+ ions.
In summary, Sr6YSc(BO3)6:Ce3+,Tb3+ phosphors was synthesized successfully by the conventional high temperature solid-state reactions. The luminescence spectra and decay curves demonstrated that Ce3+ ion can absorb UV photons via the allowed 4f-5d absorption and greatly enhance the green emission of Tb3+ ion under the 365 nm excitation. Sr6YSc(BO3)6:Ce3+,Tb3+ shows obvious absorption peaks including the absorption lines of Ce3+ ions and Tb3+ ions. Due to existence of the energy transfer, the emission hue of the phosphor can be varied from blue (0.1682,0.0934) eventually to bluish green (0.2243,0.3261) with increasing Tb3+ concentration and a fixed Ce3+ content in Sr6YSc(BO3)6. Thus, the obtained Sr6YSc(BO3)6:Ce3+,Tb3+ phosphor is expected to be developed as a suitable phosphor candidate for the application in near-UV excited white LEDs.
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This present work was supported by the National Natural Science Foundations of China (Grant No. No.51002146, No.51272242), Natural Science Foundations of Beijing (2132050), the Program for New Century Excellent Talents in University of Ministry of Education of China (NCET-12-0950), the Fundamental Research Funds for the Central Universities (2011YYL131), Beijing Nova Program (Z131103000413047) and Beijing Youth Excellent Talent Program (YETP0635).
Authors and Affiliations
School of Materials Sciences and Technology, China University of Geosciences, Beijing, 100083, PR China
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